The present invention relates generally to charge pumps and more particularly to charge pumps that operate at low voltages by utilizing distributed charge boosting.
The popularity of portable electronic devices has substantially increased demand for smaller, lighter, longer lasting portable devices. Thus, a major trend in the manufacture of laptop computers, cell phones, and other portable, battery-powered devices is toward a reduction in the voltage levels required to operate the integrated circuits which are used in the various components of those devices.
In order to reduce power consumption and extend battery life, much of the integrated circuitry used in portable devices is being designed to run at low voltage levels. This reduces the power usage and reduces the heat generated by the circuit components allowing more components to be placed closer to one another. The circuitry and components used in portable computers typically are being designed to operate at voltages levels substantially less than the previous standard of 5V, with 1.0V and lower becoming increasingly common.
However, the desire to not compromise the number and quality of features in portable devices as compared to their non-portable counterparts has led to an increase in the number of circuits used, thus requiring more power. These circuits still require higher voltages to function properly.
An example of a function that requires higher voltages relates to the basic input/output system (BIOS) information of a computer. As improvements in a computer or its peripherals are developed, the BIOS information typically stored in a read only memory (ROM) device or similar circuit providing a non-volatile read only memory needs to be updated. Historically, such changes had to be accomplished by physically removing the ROM and replacing the old circuit with an entirely new circuit having the new BIOS information. The expense and the considerable complexity involved in such procedures made this undesirable for normal computer users.
More recently, electrically erasable programmable read only memory (EEPROM) has been used to store BIOS information. This type of non-volatile memory device can be reprogrammed by running a small update program without removing the circuitry from the computer. Running the update program to reprogram the EEPROM requires approximately 12-16 V for erasing and writing operations. The voltages provided in the batteries of portable computers must be boosted for such operations.
Another example involves Flash EEPROM devices arranged in large arrays to mimic hard disk drives. Flash EEPROM arrays provide a smaller and lighter functional equivalent of a hard disk drive which operates more rapidly and is less sensitive to physical damage. Such memory arrays are especially useful in portable computers where space is at a premium and weight is extremely important. However, Flash EEPROM arrays also require much higher voltages for writing and erasing data than can be provided directly by the batteries of most portable computers and it is necessary to generate voltages greater than the device supply voltage and/or voltages more negative than ground.
In such instances, where the lower voltage batteries being employed in portable electronic devices are unable to provide a sufficiently high voltage to operate a device or certain circuitry by itself, a xe2x80x9ccharge pumpxe2x80x9d or xe2x80x9cbucket brigadexe2x80x9d circuit has typically been used to generate a higher voltage from the available lower voltage. Such circuits shift electrical charge along a series of diode-connected transistors stages that are driven by capacitively coupled clock drivers, typically two-phase clocks, to boost voltage. The source of the charge, a low voltage battery for example, introduces the charge at one end of the pump and it is shifted along and its voltage is increased until the desired voltage is reached at the output.
A major problem is that conventional charge pumps have difficulty dealing with the lower battery voltages being used. In particular, the MOS transistors used in the charge pumps have switching threshold voltages that are a large fraction of the supply voltage. The problem is related to the fact that diode-connected transistors develop increasing back-bias between the source and the body of the transistor as the voltage increases along the length of the pump. The result of this back-bias (also known as the xe2x80x9csource-body effectxe2x80x9d, xe2x80x9cM factorxe2x80x9d, or xe2x80x9cbody effectxe2x80x9d) is to increase the effective threshold of the transistor, in some higher voltage cases almost doubling it. With increased effective transistor thresholds and decreased supply voltages, the charge pump transistors would no longer switch properly and the charge pump would not work.
Many designs used a technique called xe2x80x9cbootstrappingxe2x80x9d to generate higher amplitude clock signals to compensate for the increased effective threshold voltages relative to the supply voltage. The bootstrapping technique involves the use of a charge capacitor that charges on every clock pulse and discharges between pulses, adding the discharged voltage to the original input voltage of the bootstrapping circuit so the output could be multiplied to a number of times the original input. Applying a uniform high clock voltage, generated by bootstrapping, leads to energy inefficiency because the greater the current delivered by the clocking voltage, the less efficient the bootstrapping operation. In the latter stages where high voltages are required, this inefficiency was unavoidable. In the initial stages of the charge pump, where as high a voltage is not needed, the clock bootstrapping operation was inefficient.
In general, currently available charge pumps are inefficient, large, and complex. They do not properly deal with low initial supply voltages and fail to address the problems inherent with higher threshold voltages caused by the body effect. A solution, which would provide a simple charge pump with efficient operation using a low initial supply voltage, has long been sought but has eluded those skilled in the art. As the popularity grows of portable battery-powered devices in which such a design could be particularly useful, it is becoming more pressing that a solution be found.
The present invention provides a charge pump system, including a charge pump and associated distributed clock generation circuitry, for generating high voltages from a low initial voltage in applications such as erasing and programming electrically erasable programmable read only memory (EEPROM) arrays. The charge pump system uses a power supply voltage and a clock and includes a first phase bootstrapping circuit, an inverter, and a second phase bootstrapping circuit, and a two-stage charge pump. The two bootstrapping circuits are both responsive to the clock and use a distributed bootstrapping scheme to provide first and second phase clock signals with fixed multiples of the power supply voltage in order to overcome increased transistor threshold voltages, increase efficiency, and allow for charge boosting in a limited number of stages. The inverter is connected to the second phase bootstrapping circuit, causing the second phase clock signal to be opposite in phase from the first phase clock signal. The two-stage charge pump is responsive to the power supply voltage and the first and second phase clock signals. A high voltage is produced from the two-stage charge pump by alternately adding charge to the power supply voltage in each cycle of the first and second phase clock signals.
The present invention furthermore provides a charge pump system in which the first and second phase bootstrapping circuits provide high amplitude clock signals in order to overcome transistor threshold voltages increased due to the body effect. This allows for optimized clock signals and results in quick and efficient operation.
The present invention furthermore provides a charge pump system wherein native transistors are used for a decreased transistor threshold voltage. Such a lowered threshold voltage aids the circuit in operating with lower initial voltages.
The above and additional advantages of the present invention will become apparent to those skilled in the art from a reading of the following detailed description when taken in conjunction with the accompanying drawings.